Long queues of voters were seen in the four constituencies - Darjeeling, Jalpaiguri, Alipurduar and Coochbehar - since polling began at 7 a.m., kick-starting the staggered five-phase balloting in the state, an official said.

Simultaneous by-polls are being held in the Kumargram and Maynaguri assembly constituencies in Jalpaiguri district.

Polling is scheduled to end at 6 p.m.

"The turnout till 3 p.m. ranged from 70 to 73 percent in the four constituencies," an official said.

Alipurduar maintained its lead at 72.6 percent closely followed by Jalpaiguri at 72. Coochbehar recorded a turnout of 69 percent followed by Darjeeling at 68.6 percent.

Meanwhile, the All India Forward Bloc accused the ruling Trinamool Congress of unleashing terror and adopting unfair means, including booth capturing in several places in Cooch Behar.

Party candidate Dipak Roy protested before the district magistrate and said the party will approach the Election Commission seeking repolling in three assembly segments - Natabari, Cooch Behar South and Sitalkuchi.

The Trinamool denied the allegations.

"The body language of the Forward Bloc candidate shows he is heading for a defeat. It is a known ploy, that when you know you will lose, you start making allegations," said North Bengal Affairs Minister Gautam Deb.

Special election observer Sudhir Kumar Rakesh said: "Whatever complaints we received, we have passed them on to the observers and the district administration. They are looking into it. Till now, no major discrepancy has come to our notice."

Around 120 companies of paramilitary forces have been deployed.

A total of 6,033,310 voters, including 2,909,975 women, are eligible to exercise their franchise to make their choice from among 47 candidates.

The Trinamool, Left Front, Congress, the BJP and the Bahujan Samaj Party are contesting from all the seats.

Darjeeling - for long a political flashpoint over the issue of a separate Gorkhaland state - is witnessing a bitter fight between the Trinamool and the BJP, the latter again enjoying the endorsement of the prominent hill-based party Gorkha Janmukti Morcha.

The main battle is between BJP's S.S. Ahluwalia and football icon Bhaichung Bhutia of the Trinamool Congress, with the Communist Party of India-Marxist nominating Saman Pathak, and the Congress naming Sujoy Ghatak.

West Bengal has 42 Lok Sabha constituencies. The next four phases of polling will be held April 24 (six constituencies), April 30 (nine seats), May 7 (six seats) and May 12 (17 seats).